Magnetically anisotropic bodies having a concentration gradation of material and method of making the same



Feb. 22, 1966 G. HEIMKE a-rm. 3,236,700

MAGNETICALLY ANISOTROPIC BODIES HAVING A CONCENTRATION GRADATIOH OF MATERIAL AND METHOD OF MAKING THE SAME Filed June 13, 1963 PRESSING DIRECTION CRISTALLOGRAPHICALLY AND MAGNETICALLY PREFERED DIRECTION United States Patent 3,236,700 MAGNETICALLY ANISOTROPIC BODIES HAVING A CONCENTRATION GRADATION OF MATE- RIAL AND METHOD OF MAKING THE SAME Gunther Heimke, Buschdorf, near Bonn, and Erich Steingroever, Bonn, Germany, assignors to Magnetfabrik Bonn G.m.b.H. vorm. Gewerkschaft Windhorst, Bonn, Germany Filed June 13, 1963, Ser. No. 287,630 4 Claims. (Cl. 148-3157) This invention relates to magnetically and crystallographically anisotropic permanent magnets and to a method of making the same.

It is known to make magnets from alloys which, in addition to iron, contain essentially cobalt (-30%), nickel (l1.5%), and aluminum (611%). Besides said elements, also Ti (0-6% Cu (07% V, Nb, alone or combination, and small amounts of other metallic impurities may be present. Said magnets are known as Alnico magnets and can be roughly divided into four large groups:

(a) Alnico magnets having magnetic properties which are the same in all directions (isotropic Alnico magnets). They have a (BH), value up to about 2x10 gaussoersted;

(b) Alnico magnets in which one direction has been magnetically preferred by a suitable heat treatment in a magnetic field. Such anistropic Alnico magnets have a (BH) value up to about 5.5x l0 gauss-oersted, measured in the principal direction;

(c) Alnico magnets which receive first a cristallographic preferential direction so that the major part of the crystallites lies parallel with one each of their (100) directions (columnar crystallization) and which subsequently have been subjected to a heat treatment in a magnetic field, whereby the magnetic and crystallographic preferential direction coincide. With such magnets, values up to (BH),,,,,,,=9-10 gauss-oersted can be reached.

(d) Alnico magnets which consist of a monocrystal and in which the magnetic preferential direction is parallel to a (100) direction of the crystal. Such crystals, have (BH) values up to 12-10 gauss-oersted.

Magnets of the groups (a), (b), and (c) are being manufactured commercially in large amounts while those of the group (d) have been made so far only on a laboratory scale.

Magnets of groups (a) and (b) have been prepared by casting as well as by sintering. Magnets of group (c) have been made heretofore only by casting whereby the columnar crystallization in such magnets is obtained by producing during solidification of the melt at temperature gradient which is as high as possible.

Magnets of group (d) can be drawn from the melt only with particular precautions.

It is a principal object of this invention to produce columnar crystallization in Alnico magnets prepared by sintering techniques.

Other objects and advantages will become apparent on consideration of the specification and claims.

Though applying a temperature gradient produces columnar crystallization in cast magnets, this method was not successful for sinter magnets, and other physical magnitudes have been attempted to obtain such crystallization also in sinter magnets, which mostly were also without success.

In accordance with this invention, a preferred direction of crystallization is obtained by introducing a concentration gradient as follows:

Several mixtures of Alnico alloy powders are produced which differ in the content of at least one of the essential alloy components. The concentrations of said selected component must not deviate too far from the magnetic optimum composition. For instance, if the optimum concentration of cobalt for the production of magnetically valuable alloys is 24%, powdery mixtures containing 20, 21, 22, 23, 24, 25, 26, 27, and 28 percent of Co, respectively, may be prepared.

Said mixtures are placed successively into the mold so as to produce regular drops in concentration of equal value. Thereby, it is of advantage to subject separately each mixture, after it has been placed in the mold, to 4 to of the conventional pressure before the mixture of the next following concentration is placed thereon. Finally, the pressure conventionally employed for the production of such sintered magnets is then applied to the combined pre-compressed layers. The thus prepared sintered bodies are then processed in conventional manner by methods as described, for instance, in Patent No. 2,192,741. The magnetic values of the end products are between 5.9 and 7.6 X 10 gauss-oersted.

The following example is given to illustrate the method of the invention.

EXAMPLE 1 Five powdery metallic mixtures were prepared. which The C0 content was different and was 22, 23, 24, 25 and 26%, respectively. The balance in each mixture was iron. The aluminum was introduced in form of a cobalt-aluminum prealloy, and additional cobalt was added to make up the contents recited above.

A mold of 10 mm. diameter was used to make the sintered body. First, 2 g. of the 22% Co mixture was introduced in the mold and compressed with a pressure of kg./cm. Subsequenlty, the ram was removed from the mold cavity, and 2 g. of the 23% Co mixture were introduced and compressed with 100 kg./cm. This procedure was repeated for each mixture until the 26% Co mixture had been introduced. Then a pressure of 5 t/cm. was applied, and the molded body was taken out of the mold. Ten molded magnets were prepared in the same manner and sintered in a vacuum furnace at a pressure of l0 Torr and at a temperature of 1350 C. for 2 hours. Heating of the samples to the end temperature, and cooling to room temperature, was carried out in the furnace while said vacuum was maintained. After the vacuum treatment, the samples were heated in air to 1260 C., kept for 10 minutes at said temperature, and brought into a magnetic field whose direction was parallel to the axis of the magnets. Thereby, the magnetic field strength was 1200 oersteds. In said magnetic field, the magnets cooled down to 600 C. within half an hour. Then they were :removed from the magnetic field and cooled to room temperature. For annealing, these magnets were maintained for 2 hours at a temperature of 620 C. and subsequently for 10 hours at 560 C.

After cooling to room temperature, the samples were ground parallel at their front faces and otherwise round without peaks. The magnetic values measured in a magnet steel testing device Permagraph were 7.6 10 gauss-oersted.

In similar manner, an arched columnar crystallization can be produced whereby the concentration gradient must be curved conforming to the desired curvature of the columnar axes. This modification of the invention will be explained with reference to the accompanying drawing in the following example.

3 EXAMPLE 2 The five powder mixtures described in the preceding example were compressed sequentially with a pressure of 200 kg./cm. to segments. Said segments were placed together in the manner and form shown in the drawing and then consolidated to a unitary molding by applying pressure of 5 tons/cm. in the direction of the arrow. The thus produced moldings were then processed in the same manner as set forth in Example 1. The magnetic field in which the sintered magnets were heat-treated, had the same curvature as the preferential direction indicated in the drawing. Exact magnetic measurement of the thus produced magnets could not be made as such curved magnets do not satisfy the conditions for such measurements, namely homogeneity of induction and field strength in the respective cross section. However, a magnetic circuit made from said magnets provided an air gap induction which was percent higher than that of magnets made by known methods.

While we have described. the invention with respect to a regular increase of the concentration of a single component of the alloy, similar concentration gradients may be applied to two or more of the alloy components, whereby preferably the central layer has the same concentration as the desired mean concentration of the combined layers. When varying the concentration of several components, antiparallel gradients may be produced by varying the concentrations in opposite directions.

We claim:

1. A method of making a sintered magnetically anisotropic permanent Alnico magnet containing as essential ingredients cobalt, nickel, and aluminum, said method comprising providing a series of Alnico powder mixtures containing one of said ingredients in a concentration which increases in substantially equal steps from the first to the last mixture of said series, compacting said series of mixtures to a multi-layered body whose concentration of said essential ingredient increases in substantially uniform steps from one end to the opposite end, sintering said body, and subjecting said sintered body to a uniform heat treatment in a magnetic field in the direction of said concentration gradient.

2. The method as claimed in claim 1 comprising arranging said successive layers of gradually increased concentration to a curved. body, thereby producing a corresponding curvature of the preferential direction of crystallization.

3. The method as claimed in claim 2 comprising conforming the direction of said magnetic field to the curvature of the preferential direction of crystallization.

4. A sintered magnetically anisotropic permanent Alnico magnet containing as essential ingredients cobalt, nickel, and aluminum and having a curved preferred direction of crystallographic and magnetic orientation, the concentration of one of said essential ingredients increasing in substantially equal steps along said preferred direction from one end to the other end of the magnet.

References Cited by the Examiner UNITED STATES PATENTS 1,775,358 9/1930 Smith 29-1822 1,896,853 2/1933 Taylor 29182.21 2,617,723 11/1952 Studders et al 148101 2,652,520 9/1953 Studders 208 2,837,452 6/1958 De Vos et a1. 148-31.57 3,085,036 4/1963 Steinort 148-3157 3,097,329 7/1963 Siemens 29-182.2 3,148,981 9/1964 Ryshkewitch 75-208 DAVID L. RECK, Primary Examiner. 

4. A SINTERED MAGNETICALLY ANISOTROPIC PERMANENT ALNICO MAGNET CONTAINING AS ESSENTIAL INGREDIENTS COBALT, NICKEL, AND ALUMINUM AND HAVING A CURVED PREFERRED DIRECTION OF CRYSTALLOGRAPHC AND MAGNETIC ORIENTATION, THE CONCENTRATION OF ONE OF SAID ESSENTIAL INGREDIENTS INCREASING IN SUBSTANTIALLY EQUAL STEPS ALONG SAID PREFERRED DIRECTION FROM ONE END TO THE OTHER END OF THE MAGNET. 